The Coffin of Time is a forbidden magic that stops time. For every round that the afflicted wishes to stop time, a successful intelligence check at a minus 5 must be made. If failed, time resumes and the user gets a penalty of 1 to their intelligence until after a long rest.
While time is stopped, the user may still age and complete actions as they wish. They may cast a spell each round, but none of them go off until time resumes. If used effectively, a caster could, for an example, essentially cast five lightning bolts in a round, dealing anywhere from 15-300 points of damage.
Every failure of the intelligence check reduces the afflicted's intelligence by 1. Each long rest regains 1 point. If the caster used Chrono Coffin five times in a day, eventually failing the intelligence check each time rather than ending it voluntarily, it would take upwards of five days to regain all the lost intelligence points.